Method and furnace for maintaining the temperature level of metal melts

ABSTRACT

A holding furnace comprising a crucible having a cover, an induction coil around the crucible and a gas permeable guiding means in the furnace above the melt level for introducing a protective gas into the sealed furnace.

United States Patent Gesek et al.

1 June 25, 1974 METHOD AND FURNACE FOR MAINTAINING THE TEMPERATURE LEVEL OF METAL MELTS Inventors: Rudolf Gesek; Walter Fadler, both of Vienna, Austria; Georg Bohm, Heddesheim, Germany Assigne ez ETi fiUnion ATC. Tu? eTlitrisc lie Industrie, Wien, Austria and Badische Anilin- & Soda Fabrik A. G., Ludwigshafen, Germany Filed; Apr. 23, I972 Appl. No.: 246,545

US. Cl. 13/27 Int. Cl. H05b 5/00 Field of Search 13/26, 27, 29, 31

[56] References Cited UNlTED STATES PATENTS 3,056,847 10/1962 Junker 13/27 3,095,464 6/1963 Tagliaferri 13/29 Primary Examiner-Roy N. Envall, Jr. Attorney, Agent, or Firm-Cecily L. Frey 5 7] ABSTRACT A holding furnace comprising a crucible having a cover, an induction coil around the crucible and a gas permeable guiding means in the furnace above the melt level for introducing a protective gas into the sealed furnace.

14 Claims, 6 Drawing Figures SHEET 3 OF 3 PATENTEDJUN 25 1974 METHOD AND FURNACE .FOR MAINTAINING THE TEMPERATURE LEVEL OF :MELTS The invention relates to a method andafurnace for maintaining the temperature level of metal melts of specified compositions, particularly ofcast steel, which require preventing an oxidation loss of. alloying components and an increase ingas content during theholding or maintaining period. The invention also relates sto temperature maintaining furnaces (in the following called holding furnaces) forperforming the .proposed method.

A method is already known forpurifyingthe'melt in a metallurgical induction-type furnace without the application of vacuum, which comprisesintroducing an inert gas for stirring the melt below the moltensurface. This earlier method concerns a treatment of special high-alloy steels, comprising agitating of the melt'by the inert gas introduced from .thebottom.

However theemployment of this method for keeping a melt hot for many hours is impractical for-.economic reasons alone since excessivelylarge volumes of gas would have to be supplied and consumed. vDueto the turbulence that is generated by the inert gas entering from below in a powerful stream for. agitating the melt or bath, sometimes also entering through a pipe from above, an immediate and complete intermixture of the atmospheric air originally present above the melt surface with the inflowing inert gas will occur, and the production of a pure inert gas atmospherewould require the provision of at least ten to twenty timesthe quantity of noble gas consumed in the method according to the present invention.

On the other hand, for the protection of bulkier castiron parts, large holding furnaces are usually provided which can be charged from small melting furnaces until the amount of molten metal required for pouring-has been collected. This saves the capital expenditure involved in the provision of a large and expensive melting furnace, a fact that is of particular advantage when large castings are only occasionally needed.

Moreover, foundries possessing only small-capacity melting furnaces are thus, nevertheless, :equipped to produce large castings. At the same time, thisconventional method of holding the temperature of melts offers the possibility of producing shaped castings in mechanized or automated plants since a continuous supply of liquid material is made available.

During the collection of a cast-iron melt in a holding furnace, the melt is exposed to the atmosphereso that oxidation may cause the composition of the melt to change. in a cast-iron melt having the usual carbon content of about 3.5 percent by weight, oxidation loss amounting to about 0.1 percent carbon caused by atmospheric oxygen is not of considerable significance, and the liquid cast iron can remain in a holding furnace for prolonged periods without undergoing detectable or undesirable changes.

This method which as such is advantageous for the production of iron castings cannot beused for cast steel because the latter has asubstantially lower carbon content than ordinary cast iron, namely in the order of only 0.1 to 0.2 percent. Any oxidation loss of alloying elements during the holding period would therefore reduce the carbon content of the molten cast steel to a level representing only a fraction of the required carbon content.

in the same-way, other alloying.additions, such as silicon. and the-like, would beattackedby the atmosphere which'would also introduce hydrogen and oxygen into the-'meltandallow these elementsto react with components of the melt. Due to all thesefactors a cast-steel melt that had been kept hotinatmospheric air for a :longer'period of time would no longerrhave the speciparts were notavailable for steel castings.

The-present invention providesa method for maintaining the temperature-level of metal melts of specifiedcompositions, for example steelmelts, The method comprises the steps of, a closing a furnace after a first batch of melt has beenfilled into the furnace; b

.slowly.displacing the atmospheric air from above the metal melt surface by introducing in laminar flow, without .theggeneration of turbulence, a gas that functions as aprotective gas by being inert to the melt, preferably in volumes equal to at least 1.3 times the volumetric capacity of the holding furnace, more preferably'2.5 to 5 times this capacity; and c thereafter maintaining,

,even after further batches of melt have been charged, .a pure protective gas atmosphere in the furnace interior, above the melt surface, at an excess pressure of 0.05 to 0.5.atmospheres. This is done by blowing in more protective gasto meet a consumption of between 0.l and 1 percent of the volumetric capacity of the holding furnace per minute, thereby preventing atmospheric air fromgaining access to the melt, and permitting the latter to be cast in one or several fractional pours, or cast continuously, without oxidation of the alloying components and without increase in its gas content, even hours after having been melted down.

According to animportant featureof the invention, the protective gas is introduced in laminar flow during .thed splacement of th t sph c a PI of about 0.5 atmospheres excess pressure, whereas the protective .gas atmosphere is subsequently maintained by the application of an excess pressure of about 0.5 atmospheres.

Argon or some other noble gas that is heavier, than air may be used as protective gas.

The introduction in laminar flow of the protective gas may be effected, according to another important feature of the-invention, just above the surface of the bath or melt through the furnace wall or from below the bottom edge of the furnace cover.

A holding furnace according to the invention is also provided in which at least one gas-permeable brick, whichmay be cone-shaped or annular and pressed into its seat, or at least onegas-pressure reducing chamber resemblingan exhaust silencer is located in the furnace wall above the highest level of the melt and provided 1 with .a gas-supply duct.

with a gas-supply duct, the lid having a central charging opening which can be closed with a second cover.

Holding furnaces within the scope of the invention may also be characterized in that a cover is provided with an elevationally adjustable gas-supply pipe in a sealing tube, the bottom end of said pipe carrying a preferably pot-shaped, gas-permeable and refractory flushing device or a funnel.

Finally, it is also within the scope of the present invention to provide a tubular pouring spout that can be closed with a cover, or a pouring spout containing a partition creating a syphon and, if desired, an additional outlet for the protective gas.

Other objects and many of the attendant advantages of the invention will be readily appreciated as the same becomes better understood by reference to the following detailed description, when considered with the accompanying drawings, wherein FIG. I shows, in a longitudinal section, an induction type crucible furnace in which the protective gas is introduced from the furnace wall, closely above the highest level of the bath or melt, through an annular gaspermeable refractory brick located near the underside of the furnace cover;

FIG. 2 shows a modified embodiment of the top part of the furnace of FIG. 1;

FlG. 3 is a schematic illustration, partlyin section, of a tiltable barrel-type furnace fitted with an externally located inductor, in which the gas supply is through expansion chambers provided in the uppermost part of the furnace walls;

FIG. 4 is a furnace of the same type as shown in FIG. 3, in which the gas supply is provided through chambers built into the furnace cover; and

FIGS. 5 and 6 show elevationally adjustable pipe means for the introduction of the protective gas, closely above the existing level of the melt, compatible with any of the furnace embodiments of FIGS. 1 through 4.

With reference first to FIG. 1, a crucible ll of the furnace 10 is mounted on base members 8 and 9. The crucible has a vertical axis 11a and is surrounded by an induction coil 12. A cast-steel melt or bath 13 is contained in its furnace chamber 14, the highest level of the bath being at 13a. The space 15 above the melt surface is sealed by a cover 17 placed over the end of a pouring spout l6, and a furnace cover 18 closes the top of the crucible 11. Through a gas-supply pipe 19, an inert gas is forced at an excess pressure of about 1.5 atmospheres through a gas-permeable refractory brick llb set into the crucible wall just above the maximum level 13a of the melt and pressed down on its conical seat 11c.

Passing through this brick llb, the gas enters the space 15, which atfirst contains atmospheric air, in laminar flow. If a noble gas heavier than air, preferaby argon, is used as an inert gas, the atmospheric air will be gradually displaced upwardly through the joints of the cover l8 by a cushion of heavier gas that gradually builds up from the surface 130 of the melt in upward direction. Naturally some slight turbulence and hence mixing of the noble gas with the air cannot be entirely avoided bu the boundary layer can nevertheless be kept fairly thin if the condition of entry of the noble gas in laminar flow is adhered to.

This is a necessary condition for an economically acceptable employment of a protective gas, and in the most favorable circumstances the volume of protective gas that must be used during the period of air displacement will be 1.3 times the volumetric capacity of the furnace, though in the nonnal case between 2.5 and 5 times this volume must be introduced for a pure protective gas atmosphere to be established and later maintained above the melt level in space 15. However if turbulent conditons arise when the protective gas is being introduced, then the gas volume that is needed will rise to at least 10 times this value.

Following the period of air displacement, which in the case of a furnace for 1 metric ton of molten cast steel takes between 15 and 40 minutes, the introduction of protective gas is continued at an excess pressure of about 1.5 atmospheres, there being a continuous gas consumption per minute of 0.1 to 1 percent of the furnace capacity, and a protective-gas excess pressure of 0.05 to 0.5 atmospheres will become established in the space 15. In the case of larger-capacity furnaces, this consumption is relatively lower than in smaller furnaces because large-capacity furnaces can be better sealed than small furnaces. Large-capacity holding furnaces therefore prove to be economically superior.

In the case of the modified furnace 20 shown in F IG. 2, the general conditions are quite similar to those already described but in this embodiment a cover 28 has a special central charging opening 28a which can be closed with a second, smaller cover 28b. Morevoer, the furnace cover 28 carries a ring-shaped gas-supply pipe 29 for feeding gas to a gas-permeable refractory brick 21b which is concentric with the crucible axis 210, and which is pressed down on a seat 210 so that it can serve as a bearing element for the center part of the furnace cover 28.

The gas admitted to the annular brick 21b in laminar fiow sinks to the surface 23a of the melt and slowly displaces the air in the upward direction. Parts of the modified furnace designated bynumerals 21 through 27 are quite similar to those described with reference to FIG. 1, namely parts 11 through 17 thereof.

The tiltable cylinder furnace 30 shown in FIG. 3 is provided with a lining 31 and has a temperature holding inductor ductor 32 preferably attached to the righthand lower part of the cylinder. The cast-steel melt 33, of which 33a is the maximum permissible level, fills about two-thirds of the furnace chamber 34. The space 35 above the level 33a of the melt, which is first filled with atmospheric air, is closed, on the one hand, by a partition 37 which acts as a syphon in a pouring spout 36 and, on the other hand, by a furnace cover 38. This furnace is discharged by rotatably tilting it to the left.

Gas-supply pipes 39 feed gas to preferably annularly disposed gas-pressure reducing chambers 310 set into the walls of the lining 31 of the cylinder furnace 30, closely above the highest level 33a of the melt. From these chambers 31a, which function like an exhaust for deflecting the expanding stream of gas, the protective gas spreads, preferably by the interposition of, baffles (not shown in the drawings), for deflecting the expanding stream of gas, in laminar flow across the surface 33a of the melt and gradually builds up a cushion which displaces the air.

Since in the embodiments shown in FIGS. 3 and 4, the latter to be described somewhat later, a small portion of the bath surface would be exposed to the atmosphere in the pouring spout 36 unless special precautions are taken, an additional outlet 39a, which may be formed by a gas-permeable brick, may be arranged to build up a blanket of protective gas over this part of the melt surface as well. Naturally the pouring spout 36 may also be provided with a separate cover (not shown in the drawings).

The tiltable cylinder furnace 40 according to FIG. 4 differs from the embodiment according to FIG. 3 only in that gas-supply pipes 49 and gas-pressure reducing chambers 41a are provided on, respectively in, a cover 48 of the crucible or lining 41 proper, and that the cushion of protective gas builds up as in FIG. 2. It will be understood by those skilled in the art that parts 42 through 47 and the maximum melt surface 43a are similar to those described in the previous embodiment under numerals 32 through 37 and 33a.

FIG. 5 shows a device, adjustable in height, for conveying the protective gas to a point closely above the surface of the melt (e.g. 13, 23, etc. As a matter of example, the device of FIG. 5 is shown in relation to certain parts of the embodiment of FIG. 2, consisting of a pipe 51 which is slidably mounted inside a sealing tube 52 formed on a cover 586 (similar to 28b of FIG. 2), whereas the bottom end of the pipe 51 carries a potshaped, gas-permeable and refractory flushing brick 53. Numerals 23, 23a constitute the metal melt proper and its highest level, as in FIG. 2.

In FIG. 6, the just described flushing brick is replaced by an inverted ceramic funnel 63 which functions as a gas-pressure reducing chamber. Pipe 61, for introducing the protective gas, is similar to slidable pipe Sl-of the previous embodiment.

Both arrangements are particularly suitable to be fitted into existing furnaces of the general structure as shown in FIGS. 1 and 2. The lids or covers of the exemplary holding furnaces 10, are of course easily adapted to allow the sliding and gas-tight introduction of the respective pipes 51, 61;

It will be understood by those skilled in the art that the optional devices of FIGS. 5 and 6 allow the protective gas to be introduced much closer to the melt surface, without gas losses, and they permit the comsumption of the protective gas to be minimized during the air-displacement phase. The operation is furthermore speeded up and turbulence is positively avoided by the provision of these devices.

The flushing brick 53 distributes the introduced gas. evenly over a considerable area of the melt surface, while the inverted funnel member 63 serves a similar purpose and additionally reduces the gas pressure, as applied to a unit surface of the melt, because of its outwardly flaring lower end.

The drawing is simplified in FIGS. 5, 6 in respect of conventional means for moving the elements of the devices to specified elevations, immobilizing them at a desired level, and for providing the required degree of gas-tightness, which will be self-explanatory to those familiar with this art.

It should be understood that in all embodiments described, the gas-permeable refractory bricks and the gas-pressure reducing chambers (e.g. 11b, 21b, 31a, 41a) may be located either in the furnace walls or in the covers (e.g. ll, 21, etc. and/or 17, 27; 28b, etc.) in any desired combination.

Moreover, it is also possible to take protective-gas samples and to examine them for instance by gas chromatography, or alternatively to measure the oxygen content continuously by electrical means to check for the presence or absence of air. However such measurements are largely unnecessary once the functioning of a holding furnace according to the invention has been empirically tested and comparable operations are repeated under the same conditions.

Besides argon, xenon, krypton or neon may be used as inert gas, the selection of the gas as well as the performance of the proposed method being controlled by the costs involved. In principle it would of course also be possible to use helium in which case gas outlets arranged at a high level would be beneficial.

The invention permits molten cast-steel alloys to be kept in a holding furnace according to the invention for longer periods of time, for instance for ten hours, without oxidation loss of alloying elements, so that the compositions and the technological properties of the finished castings do not deviate from the specified values. This result enables foundries to produce shaped caststeel parts in an extremely economical manner.

Moreover, the invention enables large as well as small steelworks to produce large steel castings having compositions that comply exactly with specifications, without the need for large melting furnaces. Another advantage afforded by the invention resides in the fact that air pollution is also reduced because hitherto the production of large high-quality steel castings required the provision ofa gas-heated melting furnace of corresponding size whereas the invention can be performed with relatively small furnaces which generate no exhaust gases because they are electrically heated. Finally, the invention provides a basis for considering the continuous casting of steel.

An example of performing the inventive method is given in the following. An induction-type crucible furnace of the kind shown in FIG. 2, having a capacity of 0.1 cu.m, was filled with a CK-22 grade cast-steel melt. As soon as the melting process had been completed, the furnace was'closed and argon was introduced at an excess pressure of 2 atmospheres through the gaspermeable refractory brick 21b so that it entered in laminar flow. After the gas had been introduced for 30 minutes with the consumption of 3 cu.m (S.T.P.) of argon, the oxygen content had fallen to 0.1 percent.

During the following holding period it was possible, by continuously blowing in argon at the rate of 5 liters per minute at an excess pressure of 0.5 atmospheres, further to reduce the oxygen content to ppm and to maintain it at this level, the excess pressure inside the furnace being about 015 atm. After a holding time of 8 hours the CK-22 cast-steel melt could be poured without correction of the analysis and without further additives.

It should be understood, of course, that the foregoing disclosure relates only to preferred embodiments of the invention and that it is intended to cover all changes and modifications of the examples described which do not constitute departures from the spirit and scope of the invention.

What we claim is:

l. A holding furnace for maintaining the temperature level of a metal melt of a specified composition, comprising, in combination, a crucible defining chamber therein for receiving the melt, to be filled to a predetermined maximum level, leaving a space in said chamber above the melt, which initially contains atmospheric air and is adapted to be filled with an inert protective gas,

spout means for said crucible for discharging the completed melt, cover means for said crucible, gas-supply means for introducing the protective gas into said space above the melt, and gas-permeable means for positively guiding the protective gas from said gas-supply means into said chamber, just above the predetermined maximum level, in substantially laminar flow, without the generation of turbulence.

2. The holding furnace as defined in claim 1, further comprising inductive means for heating the melt in said crucible.

3. The holding furnace as defined in claim 1, wherein said crucible is of a substantially cylindrical shape, said spout means is substantially tubular, and further comprising a gas-tight lid for the latter.

4. The holding furnace as defined in claim 1, wherein said cover means has a charging opening therein, and further comprising a lid for said opening.

5. The holding furnace as defined in claim 1, wherein said crucible is to substantially of a barrel shape, including a lining therein and a partition substantially closing off said chamber and being adapted to provide a syphon effect with respect to said spout means.

6. The holding furnace as defined in claim 5, further comprising a lid for said spout means.

7. The holding furnace as defined in claim 5, wherein said gas-permeable guiding means includes an additional outlet in the region of said spout means to introduce the protective gas beyond said partition.

8. The holding furnace as defined in claim 6, wherein said conveying means is in the form of a pot-shaped refractory discharge member.

9. The holding furnace as defined in claim 6, wherein said conveying means is in the form of a funnel-shaped refractory discharge member which widens towards its open bottom.

10. A holding furnace for maintaining the temperature level of a metal melt of a specified composition, comprising, in combination, a crucible defining a furnace chamber therein for receiving the melt, to be filled to a predetermined maximum level, leaving a space in said chamber above the melt, which initially contains atmospheric air and is adapted to be filled with an inert protective gas, spout means for said crucible for discharging the completed melt, cover means for said crucible, gas-supply means for introducing the protective gas into said space above the melt gaspermeable means for positively guiding the protective gas from said gas-supply means into said chamber, just above the predetermined maximum level, in substantially laminar flow, without the generation of turbulence, wherein said crucible is of a substantially cylindrical shape, said spout means is substantially tubular, and wherein said gas-permeable guiding means is in the form of at least one cone-shaped refractory member pressed into a seat provided in the wall of said crucible, and a gas-tight lid for said spout means.

11. A holding furnace for maintaining the temperature level of a metal melt of a specified composition, comprising, in combination, a crucible defining a furnace chamber therein for receiving them melt, to be filled to a predetermined maximum level, leaving a space in said chamber above the melt, which initially contains atmospheric air and is adapted to be filled with an inert protective gas, spout means for said crucible for discharging the completed melt, cover means for said crucible, gas-supply means for introducing the protective gas into said space above the melt, gasperrneable means for positively guiding the protective gas from said gas-supply means into said chamber, just above the predetermined maximum level, in substantially laminar flow, without the generation of turbulence, wherein said crucible is of a substantially cylindrical shape, said spout means is substantially tubular, and wherein said gas-permeable guiding means is in the form of a substantially annular refractory member pressed into a seat associated with the wall of said crucible, and a gas-tight lid for said spout means.

12. A holding furnace for maintaining the temperature level of a metal melt of a specified composition, comprising, in combination, a crucible defining a furnace chamber therein for receiving the melt, to be filled to a predetermined maximum level, leaving a space in said chamber above the melt, which initially contains atmospheric air and is adapted to be filled with an inert protective gas, spout means for said crucible for discharging the completed melt, cover means for said crucible, gas-supply means for introducing the protective gas into said space above the melt, and gaspermeable means for positively guiding the protective gas from said gas-supply means into said chamber, just above the predetermined maximum level, in substantially laminar flow, without the generation of turbulence, wherein said crucible is substantially of a barrel shape, including a lining therein and a partition substantially closing off said chamber and being adapted to provide a syphon effect with respect to said spout means, and wherein said gas-permeable guiding means is in the form of at least one gas-pressure reducing chamber associated with said lining of the crucible and adapted to act like an exhaust silencer.

13. A holding furnace for maintaining the temperature level of a metal melt of a specified composition, comprising, in combination, a crucible defining a furnace chamber therein for receiving the melt, to be filled to a predetermined maximum level, leaving a space in said chamber above the melt, which initially contains atmospheric air and is adapted to be filled with an inert protective gas, spout means for said crucible for discharging the completed melt, cover means for said crucible, gas-supply means for introducing the protective gas into said space above the melt, and gaspermeable means for positively guiding the protective gas from said gas-supply means into said chamber, just above the predetermined maximum level, in substantially laminar flow, without the generation of turbulence, wherein said crucible is substantially of a barrel shape, including a lining therein and a partition substantially closing off said chamber and being adapted to provide a syphon effect with respect to said spout means, and wherein said gas-permeable guiding means is in the form of at least two gas-pressure reducing chambers associated with said cover means.

14. A holding furnace for maintaining the temperature level of a metal melt of a specified composition, comprising, in combination, a crucible defining a furnace chamber therein for receiving the melt, to be filled to a predetermined maximum level, leaving a space in said chamber above the melt, which initially contains atmospheric air and is adapted to be filled with an inert protective gas, spout means for said crucible for discharging the completed melt, cover means for said crucible, gas-supply means for introducing the protective gas into said space above the melt, and gasthe predetermined maximum melt level, including means for adjusting the relative height of said conveying means above the actual melt level, and sealing pipe means between said gas-supply means and said conveylence, wherein said gas-permeable guiding means in- 5 ing means.

cludes means for conveying the protective gas to above UNITED STATES PATENT OFFICE Patent No 5 819 8A2 Inventor (s) Dated June 25.197

Rudolf Gesek, Walter Fadler, Georg Bohm It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

In the Title: cancel "METHOD AND" In 001. 1 cancel "METHOD AND" in heading 1, line A, cancel "a method and" 1 9, cancel "also" 1 11/12, cancel "for performing the proposed method" 1 15, cancel "already" 1 1'7, cancel "earlier" In col. 2, line 15, cancel "method" and substitute --furnace-- 2, 1'7, cancel "method" and substitute --operation of the furnace-- In 001. 6, line 8/9, cancel. "performance" and substitute --use-- 6, line 9, cancel "method" and substitute --inventive furnace-- col. 6, line In col. 7, line 7, line 7, line [SEAL] 5h, cancel "performing the inventive method-- and substitute -use the inventive furnace-- 20, cancel "to" #7, between "melt" and "gas" make a comma RUTH C. MASON Arresting Officer C. MARSHALL DANN (ommissiuner of Patents and Trademarks UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 5.819.892 Dated June 25.1979

Invent0r(s) Rudolf Geselg, Walter Fadler, Georg Bohm It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below: In the Title: cancel "METHOD AND" In col. 1 cancel "METHOD AND" in heading 1, line i, cancel "a method and" 1 9, cancel "also" 1 11/12, cancel "for performing the proposed method" 1 15, cancel "already" 1 1'7, cancel "earlier" In col. 2, line 15, cancel "method" and substitute --furnace- 2, 17, cancel "method" and substitute --operation of the furnace-- In col. 6, line 8/9, cancel. "performance" and substitute --use-- 6, line 9, cancel "method" and substitute --inventive furnace-- col. 6, line 5h, cancel "performing the inventive method-- and substitute --use the inventive furnace-- In col. 7, line 20, cancel "to" 7, line +7, between "melt" and "gas" make a comma 7, line 62, change "them melt" to the melt Signed and Sealed this sixteenth D ay 0f December 19 75 [SEAL] A ttes t:

RUTH C. MASON C. MARSHALL DANN Arresting Officer Commissioner of Patents and Trademarks 

1. A holding furnace for maintaining the temperature level of a metal melt of a specified composition, comprising, in combination, a crucible defining chamber therein for receiving the melt, to be filled to a predetermined maximum level, leaving a space in said chamber above the melt, which initially contains atmospheric air and is adapted to be filled with an inert protective gas, spout means for said crucible for discharging the completed melt, cover means for said crucible, gas-supply means for introducing the protective gas into said space above the melt, and gas-permeable means for positively guiding the protective gas from said gas-supply means into said chamber, just above the predetermined maximum level, in substantially laminar flow, without the generation of turbulence.
 2. The holding furnace as defined in claim 1, further comprising inductive means for heating the melt in said crucible.
 3. The holding furnace as defined in claim 1, wherein said crucible is of a substantially cylindrical shape, said spout means is substantially tubular, and further comprising a gas-tight lid for the latter.
 4. The holding furnace as defined in claim 1, wherein said cover means has a charging opening therein, and further comprising a lid for said opening.
 5. The holding furnace as defined in claim 1, wherein said crucible is to substantially of a barrel shape, including a lining therein and a partition substantially closing off said chamber and being adapted to provide a syphon effect with respect to said spout means.
 6. The holding furnace as defined in claim 5, further comprising a lid for said spout means.
 7. The holding furnace as defined in claim 5, wherein said gas-permeable guiding means includes an additional outlet in the region of said spout means to introduce the protective gas beyond said partition.
 8. The holding furnace as defined in claim 6, wherein said conveying means is in the form of a pot-shaped refractory discharge member.
 9. The holding furnace as defined in claim 6, wherein said conveying means is in the form of a funnel-shaped refractory discharge member which widens towards its open bottom.
 10. A holding furnace for maintaining the temperature level of a metal melt of a specified composition, comprising, in combination, a crucible defining a furnace chamber therein for receiving the melt, to be filled to a predetermined maximum level, leaving a space in said chamber above the melt, which initially contains atmospheric air and is adapted to be filled with an inert protective gas, spout means for said crucible for discharging the completed melt, cover means for said crucible, gas-sUpply means for introducing the protective gas into said space above the melt gas-permeable means for positively guiding the protective gas from said gas-supply means into said chamber, just above the predetermined maximum level, in substantially laminar flow, without the generation of turbulence, wherein said crucible is of a substantially cylindrical shape, said spout means is substantially tubular, and wherein said gas-permeable guiding means is in the form of at least one cone-shaped refractory member pressed into a seat provided in the wall of said crucible, and a gas-tight lid for said spout means.
 11. A holding furnace for maintaining the temperature level of a metal melt of a specified composition, comprising, in combination, a crucible defining a furnace chamber therein for receiving them melt, to be filled to a predetermined maximum level, leaving a space in said chamber above the melt, which initially contains atmospheric air and is adapted to be filled with an inert protective gas, spout means for said crucible for discharging the completed melt, cover means for said crucible, gas-supply means for introducing the protective gas into said space above the melt, gas-permeable means for positively guiding the protective gas from said gas-supply means into said chamber, just above the predetermined maximum level, in substantially laminar flow, without the generation of turbulence, wherein said crucible is of a substantially cylindrical shape, said spout means is substantially tubular, and wherein said gas-permeable guiding means is in the form of a substantially annular refractory member pressed into a seat associated with the wall of said crucible, and a gas-tight lid for said spout means.
 12. A holding furnace for maintaining the temperature level of a metal melt of a specified composition, comprising, in combination, a crucible defining a furnace chamber therein for receiving the melt, to be filled to a predetermined maximum level, leaving a space in said chamber above the melt, which initially contains atmospheric air and is adapted to be filled with an inert protective gas, spout means for said crucible for discharging the completed melt, cover means for said crucible, gas-supply means for introducing the protective gas into said space above the melt, and gas-permeable means for positively guiding the protective gas from said gas-supply means into said chamber, just above the predetermined maximum level, in substantially laminar flow, without the generation of turbulence, wherein said crucible is substantially of a barrel shape, including a lining therein and a partition substantially closing off said chamber and being adapted to provide a syphon effect with respect to said spout means, and wherein said gas-permeable guiding means is in the form of at least one gas-pressure reducing chamber associated with said lining of the crucible and adapted to act like an exhaust silencer.
 13. A holding furnace for maintaining the temperature level of a metal melt of a specified composition, comprising, in combination, a crucible defining a furnace chamber therein for receiving the melt, to be filled to a predetermined maximum level, leaving a space in said chamber above the melt, which initially contains atmospheric air and is adapted to be filled with an inert protective gas, spout means for said crucible for discharging the completed melt, cover means for said crucible, gas-supply means for introducing the protective gas into said space above the melt, and gas-permeable means for positively guiding the protective gas from said gas-supply means into said chamber, just above the predetermined maximum level, in substantially laminar flow, without the generation of turbulence, wherein said crucible is substantially of a barrel shape, including a lining therein and a partition substantially closing off said chamber and being adapted to provide a syphon effect with respect to said spout means, and wherein said gas-permeable guiding means is in the form of at least two Gas-pressure reducing chambers associated with said cover means.
 14. A holding furnace for maintaining the temperature level of a metal melt of a specified composition, comprising, in combination, a crucible defining a furnace chamber therein for receiving the melt, to be filled to a predetermined maximum level, leaving a space in said chamber above the melt, which initially contains atmospheric air and is adapted to be filled with an inert protective gas, spout means for said crucible for discharging the completed melt, cover means for said crucible, gas-supply means for introducing the protective gas into said space above the melt, and gas-permeable means for positively guiding the protective gas from said gas-supply means into said chamber, just above the predetermined maximum level, in substantially laminar flow, without the generation of turbulence, wherein said gas-permeable guiding means includes means for conveying the protective gas to above the predetermined maximum melt level, including means for adjusting the relative height of said conveying means above the actual melt level, and sealing pipe means between said gas-supply means and said conveying means. 